Quantum-Dot Vertical-Cavity Surface-Emitting Lasers
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Quantum-Dot
Vertical-Cavity Surface-Emitting Lasers
D. Bimberg, N.N. Ledentsov, and J.A. Lott Abstract GaAs-based continuous-wave quantum-dot vertical-cavity surface-emitting lasers (VCSELs) operating at 1.3 m at 20C with output power of 1.2 mW have been realized. Threshold currents approach 1–1.5 mA for 8-m oxide apertures. Operating voltages are 2 V. Long operation lifetimes in excess of 5000 h at 50C without degradation have been achieved. This article describes these breakthroughs, which are based on our development of complex self-organized growth technologies for defect-free stacked quantum dots. Keywords: chemical vapor deposition (CVD), compound semiconductors, crystal growth, crystal structure, electrical properties, molecular-beam epitaxy (MBE), optoelectronic materials, optical properties, photoluminescence, quantum dots (QDs), transmission electron microscopy (TEM), vertical-cavity surface-emitting lasers (VCSELs).
Introduction Diode lasers are based on current injection of nonequilibrium carriers into a semiconductor active medium, resulting in population inversion (i.e., the higher energy level becomes more populated than the lower energy level) and sufficient modal gain (an exponential increase of the intensity of the amplified emission per unit length once it propagates through the active medium) to achieve lasing. Thin or ultrathin (quantum well) layers of a narrowgap semiconductor embedded in a widegap barrier are traditionally used as the active media in such lasers. More recently, universal self-organization effects on crystal surfaces of semiconductors, which led to the spontaneous formation of coherent nanoislands,1–5 made it possible to use quantum dots (QDs) as the active media in semiconductor lasers.6 Two types of lasers currently dominate the laser market: edge-emitting lasers and vertical-cavity surface-emitting lasers (VCSELs) (Figure 1). In edge-emitting devices, the active medium (e.g., a quantum well) is placed in a waveguide having a
MRS BULLETIN/JULY 2002
larger refractive index than the surrounding cladding layers. The laser light in a narrow waveguide is diffracted at the facet at typically large angles of 30–60. The advantage of the edge-emitting laser is in its small output aperture and high power density. Highly reflective and antireflective dielectric coatings are deposited usually on the rear and front facets. In a VCSEL, the photons are cycled in a high-finesse cavity in a vertical direction; “high finesse” means that the photon bounces between the cavity edges many times before emission. The cavity is very short, and the gain per cycle is very low. Thus, it is of key importance to ensure very low losses at each reflection, otherwise lasing either will not be possible or will require current densities too large to be suitable for continuouswave (cw) operation. VCSELs were proposed as early as 1961, at the time when the idea of injection lasers using a degenerately doped p –n junction was introduced.7 The practical realization of surface lasing happened in 1979,
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